We start with that american pastime on the 4th going terribly wrong in towns across america, an estimated 14,000 fireworks show. This one turning terrifying. When a flat form collapses, the fireworks, the rockets, shooting straight at the families in the crowd. Oh e oh my god! Run! It is just incredible to listen to. Thousands who had gathered running for cover, tripping, running over one another. That then led to a frantic effort to help dozens injured in the madness. Abc's nick watt from the scene on how this happened and late word on the injured. Reporter: Those 4th of july high spirits, in the blink of an eye, turned to terror. Wow, wow, oh my god. Run, run! Reporter: Fireworks weren't exploding in the sky, they were shooting straight at the crowd, hitting spectators like shrapnel. A block of wood about this big, came shooting down and it hit the side of my leg. Reporter: Firefighters and medic treated the shocked and injured at the scene. 36 casualties in all. The youngest just 17 months old, the oldest, 71 years old. One teen was transferred to a specialist burn unit. I think it's a miracle nobody got killed. Reporter: Investigators say one device detonated prematurely, setting off a chain reaction, a domino effect, tipping over other fireworks, which fired directly into the crowd of spectators 350 feet away. A big round cylinder block hit the person sitting next to me. I think it's getting too extreme. It used to just pop. Now, I think it's getting too extreme. Reporter: Last night's disastrous display was staged by bay fireworks out of bethpage, new york. They've worked in the past with the new york mets, the republican national convention and nasa. In a statement today, bay fireworks says it "deeply regrets" the accident and the injuries and will carry out a "thorough and complete investigation and make our findings available to the public." This is where many spectators were sitting. Some suffered burns, others were hit by projectiles like this, a chunk of charred wood with a nail sticking out of it. Others trampled in the panic. 12 children injured. All of them are fine tonight, david. Just two adults remain in the hospital.

This transcript has been automatically generated and may not be 100% accurate.